1. Field of the Invention
This invention relates to a cardiac pacer for pacing a human heart, wherein the pacing rate is controlled by the respiration of the patient.
2. Related Applications:
The subject matter of the present application is related to the subject matter of the following co-pending applications filed simultaneously herewith: "A Cardiac Pacer For Pacing A Human Heart," Amundson, Ser. No. 874,588; "A Cardiac Pacer For Pacing A Heart," Elmqvist, Lekholm, Hedberg and Amundson, Ser. No. 874,597; "A Cardiac Pacer For Pacing A Heart," Lekholm and Amundson, Ser. No. 874,596; and "A Cardiac Pacer For Pacing A Heart," Lekholm and Amundson, Ser. No. 874,585.
3. Description of the Prior Art
Conventional cardiac pacers usually comprise a pacing electrode and a passive electrode. The pacing electrode is placed in the human heart by means of a pacing lead. The conductive (metallic) housing of the cardiac pacer defines the passive electrode.
The U.S. Pat. No. 3,593,718 describes such a conventional cardiac pacer which in addition utilizes an impedance pneumograph for obtaining a respiratory signal from thoracic impedance variations. The impedance pneumograph comprises a first and a second impedance measuring electrodes each connected with a separate first and second leads, respectively. Both impedance measuring electrodes of the impedance pneumograph are placed on the surface of the patient's chest.
The European Patent Application No. 0 089 014 depicts a conventional cardiac pacer which also employs an impedance pneumograph for obtaining a respiratory signal. Again the impedance pneumograh comprises first and a second impedance measuring electrodes. However, only the first impedance measuring electrode is connected to a lead while the second electrode is defined by the conductive (metallic) housing which contains the cardiac pacer and the impedance pneumograph. Furthermore, the first impedance measuring electrode and, since the conductive (metallic) housing is implanted, the second electrode are subcutaneously placed in the thorax. Under the circumstances the complete pacing and impedance measuring system comprises a pacing electrode on a first lead, a first impedance measuring electrode on a second lead and the metallic housing as both the passive electrode of the cardiac pacer and the second electrode of the impedance pneumograph. Finally the current impulses generated and utilized for impedance measurement are at a higher rate than those of the ventilation rate. Under the circumstances the pacer needs an unnecessary high amount of energy.